An example of a construction vehicle is one equipped with a so-called HST (Hydrostatic Transmission), wherein a hydraulic pump is driven by an engine and wherein a travel hydraulic motor is driven by pressure oil discharged from the hydraulic pump, thereby causing the vehicle to travel. With this type of construction vehicle, the speed and traction force of the vehicle can be controlled by controlling the engine speed, the displacement of the hydraulic pump, and the displacement of the travel hydraulic motor (Japanese Laid-open Patent Application No. 2004-144254).